I run a small media computer on my TV and often have Spotify running. The problem is that the screen gets burned in if I don’t have a screensaver, but if I have a screensaver on I can’t see the name of the song playing.
So the simple idea would be a neat screensaver that shows the current playing track. A bonus would be to show the album cover and next track, and follow the beat. As it would be a third-party application the easiest way would be to parse the artist and song title from Spotify’s Window title.
By using Gracenote it could show the album cover and lyrics, but probably not feasible for a free screensaver.
The hardest part would be inventing a cool or fun enough visualization. Maybe a self playing game, like Tetris, Audiosurf, Pong or Snake? A Rubicks cube solving itself? Maybe a nice fish tank? Or random particle effects? What would you like to see as the visualization?